The present invention relates to solid state devices for sensing the presence and mount of a gas or other targeted chemical. More particularly, this invention relates to solid state chemical sensing devices fabricated using a thin-layered diamond structure combined with one or more chemical sensitive electrodes to enhance selective detection of specific chemicals including gas concentrations of certain chemical species, concentrations of ions in liquids, and concentration of enzymes for biochemical sensing.
The first active microelectronic-based gas-sensing device was reported by Lundstrom in 1975. I Lundstrom, S. Shivaraman, C. Svensson and L. Lundkvist, "A hydrogen-sensitive MOS field-effect transistor," Appl. Phys. Lett., 26, pp. 55-57, 1975. Since then, extensive research efforts have been directed toward the development of catalytic-gate microelectronic gas sensors based on Schottky diodes, MOS capacitors, and MISFETs using silicon technology. The majority of these prior art devices have been fabricated using silicon as a semiconducting layer with catalytic metals such as palladium, platinum or other noble metal as the gate electrode for detection of hydrogen and hydrogen-containing gases. Sensitivity has been reported for hydrogen [I Lundstrom, S. Shivaraman, C. Svensson and L. Lundkvist, "A hydrogen-sensitive MOS field-effect transistor," Appl. Phys. Lett., 26, pp. 55-57, 1975; I. Lundstrom, "Hydrogen sensitive MOS-structures," Sensors and Actuators, vol. 1, pp. 400-426, 1981; T. Poteat and B. Lalevic, "Transition metal gate MOS gaseous detectors," IEEE Trans. Electron Devices, vol. ED-29, pp. 123-129, 1982; P. Ruthes, S. Ashor, S. Fonash and J. Ruths, "A study of Pd-MIS Schottky barrier diode detector," IEEE Trans. Electron Devices, vol. ED-28, pp. 1003-1009, 1981], hydrocarbons [T. Poteat and B. Lalevic, "Transition metal gate MOS gaseous detectors," IEEE Trans. Electron Devices, vol. ED-29, pp. 123-129, 1982; P. Ruthes, S. Ashor, S. Fonash and J. Ruths, "A study of Pd-MIS Schottky barrier diode detector," IEEE Trans. Electron Devices, vol. ED-28, pp. 1003-1009, 1981; U. Ackelid, F. Winquist and I. Lundstrom, "MOS structures with thermally activated sensitivity to ethanol vapor and unsaturated hydrocarbons," Proc. 2nd Ins. Meet Chemical Sensors, 1986, Bordeaux, pp. 395-398], alcohol vapor [U. Ackelid, F. Winquist and I. Lundstrom, "MOS structures with thermally activated sensitivity to ethanol vapor and unsaturated hydrocarbons," Proc. 2nd Int. Meet Chemical Sensors, 1986, Bordeaux, pp. 395-398; N. Yamamoto, S. Tonamura, T. Matsuoka and S. Tsubomura, "A study on a palladium titanium oxide Schottky diode as a detector for gaseous components," Surface Sci., 92, pp. 400-406, 1908; U. Ackelid, M. Armgarth, A. Spetz and I. Lundstrom, "Ethanol sensitivity of palladium-gate metal-oxide-semiconductor structures," IEEE Electron Device Lett., EDL-7, pp. 353-355, 1985]; hydrogen sulphide [M. S. Shivaraman, "Detection of H.sub.2 S with Pd gate MOSFETs," J. Appl. Phys., 47, pp. 3592-3593, 1976; J. P. Couput, B. Cornut, C. Chambu and S. Chouvet, "A reversible hydrogen sulphide sensitive Pd-gate MOS transistor," Proc. Int. Meet. Chemical Sensors, 1983, Fakuoka, pp. 468-472], and ammonia [I. Lundstrom, M. Armgarth, A. Spetz, and F. Winquist, "Physics of ammonia sensitive metal oxide semiconductor structures," Proc. 2nd Int. Meet. Chemical Sensors, 1986, Bordeaux, pp. 387-390; J. F. Ross, I. Robins and B. C. Webb, "The ammonia sensitivity of Platinum gate MOSFET device: dependence on gate electrode morphology," Sensors and Actuators, 11, pp. 73-90, 1987].
Research has also been conducted for the detection of other gases through the use of perforated gate structures (K. Dobos, D. Krey and G. Zimmer, "CO-sensitive MOSFET with SnO.sub.2 -, Pd-, and Pt-gate," Proc. Int. Meet. Chemical Sensors, 1983, Fukuoka, pp. 464-467), metal alloys [R. C. Huges, W. K. Schubert, T. E. Zipperian, J. L. Rodriguez and T. A. Plut, J. Appl. Phys., 62, 1074, 1987], or bilayer catalysts [W. P. Kang and C. K. Kim, "Performance analysis of a new MIS capacitor incorporated with Pt-SnO.sub.x catalytic layers for the detection of O.sub.2 and CO gases," J. Appl. Phys., 75, pp. 4237-4242, 1994]. The sensitivity and selectivity of gas sensors which use catalytic metal gates depend on parameters such as the composition and microstructure of the metal gate, and the operating temperature of the sensor.
Many interesting applications have been demonstrated and practical devices have been commercialized using microelectronic-based gas sensors. However, the relatively limited temperature operating range (&lt;200.degree. C.) of a silicon-based device has prevented the widespread utilization of these sensors, particularly for the detection of toxic gases from the combustion process and in sire emission control at high temperature. This limitation critically stifles the full exploitation of microelectronic-based devices for chemical sensing applications.
The prior art also has suggested the use of doped and intrinsic polycrystalline diamond in a solid state device which potentially could be used in gas sensing applications. The primary advantages in using polycrystalline diamond thin film structures for chemical sensing applications are wider and/or higher temperature operating range, simplicity in the fabrication process, flexibility in the choice of substrates, and compatibility with silicon microfabrication technology. Moreover, diamond films created using plasma-enhanced chemical vapor deposition (PECVD) possess many desirable material properties, including high thermal conductivity, chemical inertness, electrical stability, and compatibility with hostile environments. Therefore, microelectronic chemical sensors utilizing polycrystalline diamond technology can lead to high temperature capability, high performance, high reliability, low cost, and potentially be extended as a result of smart functionality, such as, diagnostics, self-calibration, fault tolerance and multi-sensor functions.
Recent advances in the PECVD process have resulted in the realization of high quality polycrystalline diamond films for device applications. Microelectronic devices such as Schottky diodes, and field-effect-transistors FET's [A. J. Tessmer, L. S. Plano and D. L. Dreifus, "High temperature operation of polycrystalline diamond FET," IEEE Electron Device Lea., vol. 14, no. 2, pp. 66-68, 1993; G. Sh. Gildenblat, S. A. Grot, C. W. Hatfield, A. R. Badzian and T. Badzian, "High Temperature Schottky diodes with t-film diamond base," IEEE Electron Device Lett., vol. 11, No. 9, pp. 371-372, 1990] have been fabricated for high temperature (T&gt;500.degree. C.) applications. Unfortunately, previous attempts to fabricate and operate a diamond-based chemical sensor have not produced devices having optimal performance characteristics, particularly in chemical sensitivity and selectivity. For example, a diamond-based MIS Schottky diode chemical sensor is described in U.S. Pat. No. 5,285,084, issued Feb. 8, 1994. The '084 sensor is claimed to be barrier dominated. The top metal contact in the '084 device forms a Schottky barrier of a predetermined barrier height prior to gas detection. The change in barrier height upon gas adsorption would lead to a change in I-V characteristics in the sensor, generally described in the literature as: EQU I=AA**T.sup.2 exp(-.phi..sub.b /.phi..sub.T)[exp(V/n.phi..sub.T)-1]
where k is the Boltzmann constant, T is in degrees Kelvin, A** is the effective Richardson constant, A is the junction area, n is the ideality factor, and .phi..sub.T = kT/q. Operation of the '084 device (or any other Schottky barrier chemical sensor) is therefore controlled by the thermionic emission process. Fabrication and operation of a solid state device whereby conduction of current through the sensor is barrier dominated and controlled by thermionic emission results in poor sensitivity and selectivity. The optimal mechanism for detection of a gas or other chemical in a solid state sensing structure is not to determine a shift in a characteristic Schottky barrier curve, as demonstrated by the dashed line curves plotted on FIG. 12. Consequently, a diamond-based Schottky diode structure is not preferred for high performance chemical sensing applications.
What is needed, then, is a solid state chemical sensor which is easy to fabricate, which offers high sensitivity and selectivity, and which can be reliably operated at high temperatures and under other harsh environmental conditions.